TLS中PSK的簡要介紹

PSK的目的

??我們都知道TLS需要依賴非對稱算法（RSK，EC，DS，DH...）完成秘鑰交換，身份認證的功能，但是非對稱算法的耗時和耗計算資源的特性在對資源或者耗時敏感的場景下，你就想把他優化掉。本文我們就簡紹一種TLS標準本身提供的優化方式:PSK.

PSK的江湖地位

??PSK的方式應該是最古老的一種秘鑰交換和認證方式，但是它在TLS中的江湖地位是比較低的，從最早的非正式的優化方案到有了自己的RFC編號RFC4279(December 2005)對比TLS的歷史

TLS中PSK的核心目的

  一下是RFC中的原文摘錄

This document specifies three sets of new ciphersuites for the
   Transport Layer Security (TLS) protocol to support authentication
   based on pre-shared keys (PSKs).  
These pre-shared keys are symmetric
   keys, shared in advance among the communicating parties. 

一，The first set of ciphersuites uses only symmetric key operations for  authentication.  

TLS_PSK_WITH_RC4_128_SHA           PSK           RC4_128       SHA
TLS_PSK_WITH_3DES_EDE_CBC_SHA      PSK           3DES_EDE_CBC  SHA
TLS_PSK_WITH_AES_128_CBC_SHA       PSK           AES_128_CBC   SHA
TLS_PSK_WITH_AES_256_CBC_SHA       PSK           AES_256_CBC   SHA

二，The second set uses a Diffie-Hellman exchange authenticated with a pre-shared key, and

TLS_DHE_PSK_WITH_RC4_128_SHA       DHE_PSK       RC4_128       SHA
TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA  DHE_PSK       3DES_EDE_CBC  SHA
TLS_DHE_PSK_WITH_AES_128_CBC_SHA   DHE_PSK       AES_128_CBC   SHA
TLS_DHE_PSK_WITH_AES_256_CBC_SHA   DHE_PSK       AES_256_CBC   SHA

三，the third set combines  public key authentication of the server with pre-shared key authentication of the client.

TLS_RSA_PSK_WITH_RC4_128_SHA       RSA_PSK       RC4_128       SHA
 TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA  RSA_PSK       3DES_EDE_CBC  SHA
 TLS_RSA_PSK_WITH_AES_128_CBC_SHA   RSA_PSK       AES_128_CBC   SHA
 TLS_RSA_PSK_WITH_AES_256_CBC_SHA   RSA_PSK       AES_256_CBC   SHA